1. Field of the Invention
The present invention relates to an optical system for eliminating a ghost image of a transmission optical system in a wide wavelength band.
2. Description of Related Art
For instance, in a photographic lens, even if no intensive light source (e.g., sunlight) exists in a picture plane, a ghost image can appear when a plurality of images are formed on a picture plane by light beams of an intensive light source reflected by surfaces (boundary reflecting surfaces) of a plurality of constituent lenses (transmission optical elements). In other words, a ghost image does not appear when there is no reflection of light beams by the lens surfaces. Accordingly, measures to eliminate the ghost image have been directed to a reduction in reflectance (reflectivity) of the lens surfaces. It is necessary to coat the lens surfaces with an anti-reflection layer, such as a multi-coating layer to effectively prevent reflection in a wide wavelength band. However, multi-coating is expensive and can not completely eliminate the ghost image in a wide wavelength band. Namely, the wider the wavelength band, the more difficult it is to prevent a ghost image.